The present invention relates to passive optical communications networks.
In the past ten years optical transmission systems have more and more taken over the functions of their copper counterparts in the trunk network and between central offices. However, replacement of copper based transmission in the trunk network by optical fiber based transmission is only the first step in the utilization of the large transmission capabilities of optical fibers. Indeed, deployment of optical fibers may ultimately penetrate further to the local loop plant to bring broad-band and narrow-band services directly to telephone subscribers.
Currently, one area of investigation in optical loop plants involves the use of a passive optical network (PON) in which there are no active components between a central office and the subscriber's terminal equipment. In particular, a feeder fiber from the central office provides a downstream optical signal to a remote node (RN) that splits the optical signal for distribution onto a number of optical fibers, each one of which terminates in an Optical Network Unit (ONU). The latter converts the received optical signal into electrical form and delivers it to either one or a number of subscribers. Generally speaking, there are currently two popular PON architectures under consideration for deploying optical fibers into the local loop--"Telephony over Passive Optical Networks" (TPON) and "Passive Photonic Loop" (PPL).
In a TPON architecture, the central office provides a downstream optical signal to each of the ONUs utilizing a time-division multiplexed protocol. This protocol typically comprises a frame of information that is further subdivided into time slots that are assigned to individual ONUs. Upon receiving the time-division multiplexed optical signal, each ONU then extracts the information that is contained in its assigned time slots. Consequently, each ONU must be synchronized to the transmission of the downstream optical signal to ensure that it correctly performs the demultiplexing operation. Similarly, in the upstream direction, from each ONU to the central office (through the RN), the operation of the ONU's laser must be synchronized so that each ONU transmits information only in its assigned time slot so as to avoid interfering with the optical signals provided by the lasers of other ONUs. This synchronization in the upstream direction is further complicated by the effect of the different optical path lengths, between each ONU and the RN, on an ONU's respective optical signal.
In comparison, the PPL architecture is different from the TPON architecture in that at the central office each ONU is assigned a unique wavelength, which forms the basis for segregation and routing in the downstream direction. The central office forms an optical signal for downstream transmission by "wavelength division multiplexing" the light output from a number of lasers, where each laser provides light at one of the assigned wavelengths. This wavelength division multiplexed optical signal is received by the RN where it is demultiplexed into individual optical signals, where each individual optical signal is then routed to its associated ONU. In the upstream direction, there are two variations of this PPL architecture. In the first variation, each ONU comprises a laser that transmits a light signal at its respective assigned wavelength, similar to the transmission in the downstream direction. In the second variation, instead of providing a laser with a unique wavelength to each ONU, each ONU comprises a laser in a common wavelength band. Consequently, in this second variation, transmission in the upstream direction is similar to the TPON approach described above in which synchronization of each ONU's laser is required along with a time-division multiple access protocol.
As described above, both the TPON and PPL architectures require an optical source in the ONU for transmission of information in the upstream direction to the central office. As a result, each of these optical networks must be able to control the timing and/or wavelength accuracy of the ONU's optical sources. Consequently, supervisory and timing functions reside in both the central office and in the ONU thereby increasing network vulnerability to faults and resulting in added equipment and maintenance expenses.